Try to fix null context

Signed-off-by: Mario Danic <mario@lovelyhq.com>

app/src/main/java/com/nextcloud/talk/utils/NotificationUtils.java

@@ -76,7 +76,8 @@ public class NotificationUtils {
     }
 
     public static void cancelExistingNotifications(Context context, UserEntity conversationUser) {
-        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.M && conversationUser.getId() != -1) {
+        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.M && conversationUser.getId() != -1 &&
+                context != null) {
 
             NotificationManager notificationManager =
                     (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);